1. Field of the Invention
The present invention relates to a power limiting system and method for limiting power delivered to a common bus in, for example, a one or multi-frame tape library environment. In particular, the present invention relates to a power limiting system and method where each frame is capable of delivering a limited power to a common bus, and where power delivered to the common bus is used to energize a relay in each frame. For each frame that is delivering a limited power to the common bus, a respective energized relay provides for the frame to be powered. A particular utility for the present invention is in a multi-frame tape library system, such as IBM 3584, where a large number of frames, e.g., 20 frames, may be desired but the total power supplied to a common bus must be limited to a predetermined level, e.g., 240 VA.
2. Description of Related Art
Conventional modular multi-frame tape library systems such as the IBM 3494 comprise a plurality of frames coupled together to form a tape library, for example, in a daisy-chain fashion. One switch controls a bank of relays, which, in turn, activates a frame. Typically, each frame includes the necessary hardware and software to operate those components of that frame which may include a plurality of tapes, a plurality of tape drive mechanisms for reading and writing data to tapes, and one or more controllable robot members to move tapes to and from tape drive mechanisms. The details of the tapes, tape drives, and robotic members are not important for an understanding of the present invention.
Additionally, for safety reasons, the total power supplied by all the frames should be limited to a predetermined power limit, e.g., 240 VA. Therefore, when a large number of frames, e.g., 20 frames, is added together in such systems, it is desirable for methodology in place to limit the total power supplied by the frames. Conventional techniques of limiting the total power supplied include the use of a plurality of fuses or semiconductor current limiting devices such that any point on the bus has current limiting protection which limits the available power to less than the predetermined limit. However, such techniques have some drawbacks. Fuses often have connection problems and must be replaced when they blow. Semiconductor current limiting devices are more expensive and may require more components. Both fuses and semiconductor current limiting devices add resistance to the bus wiring which affects the operation and the number of frames that can be added to the library.
Another conventional technique for limiting the total power supplied to the common bus is to separately control power delivery to each frame with an individual EPO (Emergency Power-Off) switch. With an increasing number of frames, separate EPO switches would increase the number of components required and the associate cost. Yet another conventional technique is to control power delivery to each frame using one large multi-pole EPO switch. A single multi-pole EPO switch would require additional wiring and would limit the number of frames to the number of poles of the switch.
The present invention provides a power limiting system and method for limiting power delivered by each of one or more frames to a common bus.
In an exemplary embodiment, the present invention provides a power limiting system comprising one or more frames coupled to a common bus. Each frame comprises a local DC power source coupled to current limiting circuitry. When coupled to the common bus, each frame is capable of utilizing the local power source and the current limiting circuitry to supply a limited power to the common bus.
In one embodiment, each frame further includes a relay capable of receiving power from the common bus, and capable of being energized upon receiving power from the common bus. For a frame whose local power source is on and respective relay is energized, the power limiting system of this embodiment provides for a main AC power supply to power the frame. In yet another embodiment, each frame includes a switch providing for the main AC power to power the frame when the frame""s local power source is on and respective relay is energized.
In another embodiment, the current limiting circuitry of each frame comprises a current limiting resistor. In yet another embodiment, each current limiting resistor is selectable to cause the power supplied by the respective frame to at least equal the power needed to energize a respective relay. In a further embodiment, each frame coupled to the common bus includes a blocking diode disposed between the current limiting circuitry and common bus to prevent feedback.
In method form, the present invention provides a method of limiting the power delivered by a frame to a common bus. In one embodiment, the present invention provides a method of limiting the power delivered by each of one or more frames to a common bus, such that a greater number of frames may be coupled to the common bus. In yet another embodiment, the present invention provides a method of powering only those frames, coupled to the common bus, whose local power sources are on. In a further embodiment, the present invention provides a method of energizing a relay in each frame utilizing the power supplied by that frame. In this embodiment, the present invention further provides a method of utilizing each energized relay to provide for a main AC power to power a respective frame whose local power source is on.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to exemplary embodiments and methods of use, the present invention is not intended to be limited to these exemplary embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: